Conventional golf balls can be divided into several general classes: (a) solid golf balls having one or more layers, and (b) wound golf balls. Solid golf balls typically include a solid core and at least a cover. Solid cores are generally formed using a rubber-based or elastomeric composition, typically polybutadiene or the like. The polybutadiene core material is usually processed in a two roll mill or Banbury type internal mixer, in combination with additional ingredients in order to provide adequate curing and physical property characteristics. The additional ingredients may include, free radicial initiators such as peroxides, cis-to-trans catalysts, crosslinking agents such as metal acrylates, fillers, and the like.
The processability of such a core formulation in a two roll mill or Banbury mixer, however, is affected by certain ingredients. For example, metal acrylates and their interaction with the elastomers and/or the peroxides tend to cause build-up to occur on the rolls, sides of the mixer, and on the rotors. This build-up, if not removed after every batch, generally leads to reduced heat transfer and increased cycle time. Moreover, fragments from the build-up may be mixed into subsequent batches and result in a non-uniform product.
Numerous methods have been attempted to eliminate the zinc acrylate build-up including varying the order of addition of ingredients. For instance, the conventional method of combining the core ingredients is to add most of the ingredients to the elastomer all at once and subsequently add the peroxide. To reduce the build-up from this conventional method, others have attempted to add the metal acrylate first, in order to partially encapsulate the metal acrylate and minimize contact of the metal acrylate with the other ingredients causing the build-up. However, even this order of ingredients only partially reduces the build-up and has a further disadvantage of increasing the cycle time. Another method, disclosed in U.S. Pat. No. 6,194,504, fully encapsulates the metal acrylates by contacting the metal acrylate with a solution of polymer in organic solvent, and then removing the solvent prior to compounding the encapsulated metal acrylate into the elastomeric mixture.
In addition to the build-up, the conventional dry mixing method of combining the core ingredients does not typically provide a very uniform dispersion of the metal acrylate throughout the elastomer.
Furthermore, when transferred, metal acrylates generally produce dust particles having diameters of about 10 microns, which result in odor and inhalation risks for operators. Likewise, certain cis-to-trans catalysts, such as organosulfur compounds, are also dusty in nature.
Thus, there is a need in the art for a core composition with improved processability and dispersion of the ingredients and reduced safety risks. In particular, a there is a need in the art for a core composition that includes particular ingredients in pelleted form with a processing aid to reduce the odor and inhalation problems associated with conventional core compositions and to increase dispersion of the ingredients within the elastomeric composition.